Methods exist for producing recombinant proteins using E. coli, yeast, cultured mammalian cells, plants, animals, and such. These methods all have advantages and disadvantages, and thus appropriate methods are selected according to the characteristics and intended use of the proteins to be produced.
Methods using either the nuclear polyhedrosis viruses of lepidopteran insects or transgenic silkworms have been developed as recombinant protein production systems that use insects. The former uses a DNA-type virus as the vector. Recombinant viruses can be produced relatively easily, and can then infect the larvae of silkworms or other such insects. After infection, the viruses grow in the insect individuals, resulting in recombinant protein synthesis from the genes introduced into the virus. In this method, as the virus-infected larvae die a few days later, the proteins are collected from the body fluid of the silkworm just before death, and are purified. In most cases, the amount of protein that can be produced is 1 mg or less per 1 mL of body fluid. Therefore, the purification procedure of this method is highly laborious.
Methods using recombinant silkworms utilize the silk gland, which is a unique organ of silkworms. The silk gland can be divided into the anterior, middle, and posterior regions; each has a different function. More specifically, the anterior silk gland almost has no protein-producing function, whereas 75% of the cocoon filament protein is produced in the posterior silk gland, and the remaining 25% is produced in the middle silk gland. Fibroin is produced in the posterior silk gland, and sericin is produced in the middle silk gland. Previous studies have produced posterior silk-gland-specific gene expression systems, but there are no previous successful examples of recombinant protein production from the middle silk gland.